Flip-chip packaging can generally provide a small footprint package with a large number of electric connections to an integrated circuit die.
FIG. 1A illustrates a packaged device 100 using flip-chip packaging of an integrated circuit die 110. Die 110 is an integrated circuit chip formed from a semiconductor wafer and having solder bumps 115 on an active surface. Solder bumps 115 are electrically connected to circuit elements formed in and on die 110. In packaged device 100, die 110 is flipped so that bumps 115 contact a substrate 130.
Substrate 130 is typically a printed circuit made of a material such as polyimide, polyimide alloy or compound or non alloy general polymer and metal composites; ceramic, silicon, or glass and metal composites; or similar materials forming a flexible or rigid carrier having conductive traces (not shown), which are generally made of copper or another metal. Solder bumps 115 on die 110 contact the conductive traces on the top surface of substrate 130, and the conductive traces, which extend through substrate 130, electrically connect solder bumps 115 to solder balls 135 on the bottom surface of substrate 130. Solder balls 135, which can be arranged in a ball grid array, form the terminals of packaged device 100 and can be attached to a printed circuit board or other circuitry in a product containing packaged device 100.
One concern in flip-chip packages is the difference between the coefficients of thermal expansion of semiconductor die 110 and substrate 130. This difference creates mechanical displacement stress on the connections between die 110 and substrate 130. In packaged device 100, underfill 120 between die 110 and substrate 130 strengthens the attachment of die 110 to substrate 130 to help prevent the thermal stresses from breaking the connections between die 110 and substrate 130.
FIG. 1B illustrates an edge of underfill 120. Underfill 120 contains filler particles 122 suspended in an organic resin 124. Filler particles 122 generally have a size selected according to a gap between die 110 and substrate 130, e.g., the filler particles have a diameter about one third the size of the gap. Generally, the composition and concentration of filler particles 122 are selected to control the coefficient of thermal expansion and the shrinkage of underfill 120.
Organic resin 124 that when initially applied in device 100 is a liquid that flows into the gap between die 110 and substrate 130. Accordingly, the edge of underfill 120 has a concave shape that depends on the viscosity of liquid organic resin 124 and the adhesion of organic resin 124 to die 110 and substrate 130. Organic resin 124 subsequently cures, and the presence of filler particles 122 helps control the shrinkage that occurs in underfill 120 during curing.
As shown in FIG. 1B, the distribution of filler particles 122 is relatively uniform where underfill 120 is significantly thicker than the diameter of filler particles 122. However, in fillet regions 126 and 128 where the thickness of underfill 120 approaches or is less than the diameter of a filler particle, the density of filler particles 122 falls or is reduced. The lack of filler particles 122 causes more shrinkage in fillet regions 126 and 128 during curing. This shrinkage can warp substrate 130 and disrupt electrical connections between substrate 130 and die 110 or between substrate 130 and an external circuit. In particular, shrinkage and surface tension in underfill 120 causes stress S on substrate 130 near the edge of die 110. This stress S is along a direction that depends on the wetting angle xcex1 of underfill 120 at the edge of die 110.
The lack of filler particles 122 in region 126 also makes the coefficient of thermal expansion of in regions 126 and 128 differ from the coefficient of thermal expansion in thicker regions of underfill 120. Accordingly, temperature changes can induce further stress in fillet regions 126 and 128 if the composition of underfill 120 is selected to minimize stress created by thermal expansion in thick regions of underfill 120.
To improve reliability and yield of good packages, methods and structures are sought that avoid increased shrinkage, stress that warps the substrate, and/or change in coefficient of thermal expansion that occurs at the edges of the underfill.
In accordance with an aspect of the invention, a dam, barrier, or other damming feature or discontinuity changes the shape or accumulation of the underfill material to reduce stress resulting from edge effects. In particular, the dam controls the wetting angle of the underfill material to provide a much smaller stress component perpendicular to the surface of the underlying substrate, and the underfill as shaped by the dam lacks underfill fillet regions that shrink significantly and cause stress on the substrate. The dammed underfill additionally avoids or reduces the size of areas having low filler particle concentration and thus avoids thermal coefficients of expansion that differ from the optimal coefficients. The resulting package has superior performance as defined by co-planarity, reliability, and mechanical improvement when compared to conventional overall flip-chip packages.
One specific embodiment of the invention is a packaged device that includes a substrate, a die, and a dam. The die has contacts placed as in a conventional flip-chip package so that the contacts electrically contact conductive traces of the substrate. The dam attaches to the substrate and surrounds the die to confine the edges of underfill that fills a gap between the die and the substrate. The dam controls the shape of the underfill so that wetting angles at the die and at the dam are less than 45xc2x0 or so that the underfill lacks fillet regions.
Generally, the device has a ball grid array on a side of the substrate opposite to the die. In an exemplary embodiment, the ball grid array has a pitch that is less than or about equal to one half a separation between the dam and an edge of the die. The width of the dam is typically between one and two times the pitch of the ball grid array, and the height of the dam is chosen to provide a wetting angle for the underfill that avoids stress on the substrate or areas of underfill having a low filler concentration.
Another embodiment of the invention is a method for packaging an integrated circuit die. The method includes: attaching the die to a substrate so that conductive traces on the substrate electrically contact contacts on the die; forming a dam on the substrate around the die; and filling a volume between the die and the substrate and between the die and the dam with an underfill material. The dam can be constructed before applying the underfill by placing, depositing, growing, or otherwise accumulating material on the substrate to form the dam. Alternatively, the dam can be preformed to the desired shape and attached to the substrate. The underfill is applied after the dam is in place so that the dam controls the shape and location of the edge of the underfill. Suitable materials for such dams include but are not limited to a material such as a metal layer or feature and a polymer which is filled with property modifying materials such as spheres, fibers or pieces of quartz, ceramic, or metal.
In an alternative embodiment, removing material from the substrate (e.g., by machining or etching) before a die is attached can leave a dam surrounding a die attachment area on the substrate.
In yet another alternative embodiment, treatment of the substrate increases adhesion or stiction between the underfill and the substrate in specific areas on the substrate. The underfill accumulates and can be shaped and cured to form the dam in the treated area of the substrate.